1. Field of the Invention
This invention relates to organosiloxane compositions. More particularly, this invention relates to optically clear organosiloxane compositions that cure by a platinum-catalyzed hydrosilation reaction and exhibit improved and reproducible cohesive bonding to organic polymer substrates.
2. Description of the Prior Art
The optical clarity and chemical resistance of polyorganosiloxanes make these polymers desirable for use as adhesives in a variety of optical applications. In one such application gum type organosiloxanes compositions are used to bond together sheets of optically transparent organic polymers to form composite windows and windscreens. These organosiloxane compositions can be cured by a number of methods.
One curing method involves the reaction of polyorganosiloxanes containing silicon-bonded hydroxyl groups with silanes containing 3 or 4 hydrolyzable groups per molecule. These curing reactions occur under ambient conditions and typically require the presence of atmospheric moisture. This method is not practical for a layer of polyorganosiloxane confined between two layers of organic polymer during curing.
A second curing method involves reacting a gum type polydiorganosiloxane with an organic peroxide to form an elastomeric material. The initial mixture must be heated to a temperature sufficient to decompose the peroxide and generate free radicals, which in turn initiate curing of the polyorganosiloxane. The curing reaction generates organic compounds as by-products, which could interfere with the clarity of the final cured elastomer. These by-products and the high temperatures required to initiate curing make this method less than desirable for curing optically clear organosiloxane compositions that are intended to serve as cured-in-place adhesives for organic polymer substrates.
The aforementioned disadvantages of moisture and peroxide curable organosiloxane elastomer compositions can be avoided using organosiloxane compositions that cure by a hydrosilation reaction between a polyorganosiloxane containing two or more ethylenically unsaturated hydrocarbon radicals per molecule and an organohydrogensiloxane containing two or more silicon-bonded hydrogen atoms per molecule. This reaction is catalyzed by platinum and platinum compounds and does not require moisture or the high temperatures needed to decompose many organic peroxides. While this curing method is convenient, the resultant elastomers are often less than satisfactory with respect to their ability to bond cohesively to organic polymer substrates. The prior art teaches using primer compositions and adhesion promoters of various types to improve this poor adhesion. One such primer composition suitable for improving the bonding between organosiloxane elastomers and organic polymer substrates is taught in U.S. Pat. No. 4,719,262, which issued to E. Plueddemann on Jan. 12, 1988 and U.S. Pat. No. 4,795,775, which issued to M. Baile, J. Braun and J. Clark on Jan. 3, 1989.
The present inventors found a substantial variation in the degree of cohesive bonding between primed organic polymer substrates, such as polycarbonates, and different batches of the same clear gum-type organosiloxane composition curable by a platinum-catalyzed hydrosilation reaction. Further investigations by the present inventors revealed a relationship between this variation in adhesion and the concentration of silanol groups in the various polyorganosiloxanes comprising the curable organosiloxane composition. It is known that the silanol content of ethylenically unsaturated polyorganosiloxanes can vary substantially depending upon the process conditions under which the polymers are prepared.
An objective of this invention is to provide curable elastomeric organosiloxane compositions that reproducibly provide cohesive bonding to a variety of organic polymer substrates.